MN12 wheel fitment guide

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XR7-4.6

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1994 Cougar XR7 DOHC TR3650
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When it comes to personalization, there's nothing as simple that changes the appearance of a car more than a set of wheels. Our Thunderbirds and Cougars had nice sets of wheels back in the 90s. Most were even alloy, which was still a big deal then. When it comes to modern standards in cosmetics and maximizing grip for high performance, the factory equipped 15/16" wheels and their 205-225mm can be a little lacking. While it's tempting to look at pictures of wheels and say "I want those," there's a little more to it than that. Obviously there's the physical bolt pattern which won't be covered in-depth here. The factory pattern is 5 x 108mm (or 5x 4.25"), and with a hubswap to the Mustang pattern you get 5 x 114.5 (or 5 x 4.5").

What I'll be covering in this topic is fitment and the mysterious specs of offset and its redneck cousin, backspacing. In my humble opinion, offset is the more useful measure. It's simply the distance from the wheel's centerline to the mounting surface that is fastened to the brake rotor or drum hub. Positive offset means the outside of the wheel is closer to the hub ("tucked in"), negative offset means the face of the wheel will be further away ("sticking out"). For the MN12 platform and just about any modern automobile you're only looking for positive offset. Negative and even zero offset (zero meaning the offset is the wheel's centerline) is something you're only going for find on large modified pickups and on classic cars and full size vans.

Backspacing and offset correlate directly with each other, but not often intuitively. Many wheel makers, particularly those geared toward American cars, tend to use this measure. The problem I have with it is that it only really tells you one end of things: as you can guess, the space at the back of the wheel. Remember how offset uses the one end of the measurement from the wheel mounting surface? Well so does backspacing, only rather than that point to the centerline the measurement instead goes all the way back to the inner lip of the wheel.

Consider this example, which is where it gets a little confusing. You have a wheel in mind that comes in both 17x8" and 17x9". They can actually have the same backspacing (let's say 5.95", common to many SN95 spec 9" wide wheels), but the offsets will be different. In order to find the metric offset of a backspacing you need to convert your wheel width (which is actually about 1" wider than tire neck, e.g. a 9" wheel is actually about 10" lip to lip) and backspace spec to mm (multiply by 25.4). A 17x9 would therefore be about 10" or 254mm wide, lip to lip, and the backspace is 151.13mm. Subtract that from 254 to come up with 102.87 for your "frontspacing". Remove that value from half your wheel width (half of 254mm is 127mm) and you'll come up with a +24.13mm offset. For the 17x8" wheel with 5.95" backspacing, you'll have a 36.8mm offset - that's actually quite a difference and you will notice while looking at the car!

To me, offset is the more useful metric because it's easier to visualize whether a wheel will fit well or not. This is because the total width is a constant, and if it physically fits between the fender lip and the suspension (the steering knuckle being the limiting factor on the MN12 chassis), the offset is all that needs to be scrutinized.

This illustration roughly represents a stock 39mm offset 17x7", such as the rare FRPP wheel. Note the knuckle and wheel lip clearance.

1700162599506.png


What if you want some wider meats - (255 tires on a 9" wheel) but you wanted to retain the factory backspacing?

1700162690623.png


Your backspacing remains the same but look at what happened to your fender clearance with the wider tires. Also make note of how much the offset shrunk. This is the effect you have when using wheel spacers/adapters; you're effectively reducing offset.

Now let's look at what retaining the stock 39mm offset does with the wide tire/wheel.

1700162877978.png


This increased the backspacing and your knuckle clearance is reduced (but still acceptable), and on the outside the tire is essentially flush with the fender. This seems on the ragged edge, but keep in mind that as the suspension travels, the camber gains and the control arms both lose their effective length pulling the top of the tire inboard just enough to clear.

The other thing to keep in mind is that your clearances will vary depending on your wheel diameter. On the 39mm offset 17x9" example, let's reconsider if it was a 19x9".

1700163427883.png

Note how much knuckle clearance you gained due to the larger rim's lip moving up the taper of the knuckle. You could actually use a little more wheel offset here - perhaps +42mm to gain more fender clearance.

On the flipside, if you ran a 15x9" wheel with a +39 offset, this happens...

1700163803494.png

Don't do that!


Generally speaking, through years of collected knowledge, if you want both fender clearance as well as back clearance, your maximum front tire width is limited to 255 or maybe 265 paired with a higher offset 19-20" wheel, and 295 on the rear on a 10" wheel with a +45mm offset.
 
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